We will record intracellularly from cell bodies and from axons in the monkey's visual pathway with a micropipette electrode filled with the marker enzyme horseradish peroxidase (HRP). After physiologically characterizing a neuron by determining its responses to a variety of stimuli, by mapping its receptive field and by measuring its spectral sensitivity, we will inject it with HRP. We will then be able to see its morphological characteristics and relate them to its physiological properties. Our combined structure/function studies will be concentrated in two areas: the lateral geniculate nucleus (LGN) and the striate cortex. We will record from optic tract fibers entering the LGN and map their distributions and terminations in the geniculate layers. We will also record from geniculate cell bodies and determine the morphological characteristics of the various physiological classes. By correlating the dendritic arborizations of functionally identified cells with the terminations of their different axons, we may learn something about geniculate integrative mechanisms. In the cortical studies we will examine the terminal distributions of physiologically characterized, HRP-labelled single geniculate fibers entering the monkey's striate cortex. We will compare the functional properties of different fiber types with their projection patterns. We will also investigate the morphology of individual HRP-filled color-sensitive cortical neurons to determine whether the four classes of color cells are anatomically distinctive. By studying the dendritic arborizations and the axonal projection patterns of the cortical cells, we may gain insight into the intracortical circuitry involved in color processing, into the integrative mechanisms which lead to the various response properties and receptive field types and into the significance of the layers in both the cortex and the geniculate.